It is often found that it is necessary to bore through an impervious stratum to reach an aquifer lying below it ("the lower aquifer"). This may occur because an aquifer lying above the impervious stratum ("the upper aquifer") has become polluted or depleted or both. The drilling operations carried out to access the lower aquifer are often in themselves a source of pollution because of oil, grease, drilling mud and bacteriological contamination. In addition, polluted water may seep into the lower aquifer via the annulus created between the sides of a bore hole and a bore tube inserted therein. It is normal practice to fill this annulus with grouting based on quick-setting cement, but it has been frequently observed that, very soon after drilling, the lower aquifer becomes polluted because the grouting currently used is often not completely water-tight. The perforation of an overlying impervious stratum thus risks polluting an aquifer lying beneath it.
This risk is increased when the bore hole is decommissioned, in which, conventionally, the top part of the bore tubing is pulled out and the bore hole is filled with non-watertight material, such as light concrete, gravel, and earth. At the present time, this practice is not strictly regulated.